SOHO has made the Sun transparent after five yearsESA SCIENCE RELEASEPosted: April 29, 2001

Anyone troubled by storms on the Sun will now have an extra week's early warning of eruption risks, by courtesy of the SOHO spacecraft. Teams in France and the USA have found two different ways of detecting activity on the Sun's far side, before it swings into view from the Earth. SOHO's SWAN instrument sees ultraviolet rays sweeping like a lighthouse beam across interplanetary gas beyond the Sun, while the MDI instrument peers right through the Sun to locate hidden sunspots and their active regions. From today, both teams are making their observations available routinely to everyone, including the forecasters of space weather.

The announcement of these new far-side services coincides with the celebration of Sun-Earth Day 2001, by the European Space Agency, NASA and other agencies. It also marks the fifth anniversary of the commissioning of the European-built SOHO, in April 1996, and the formal start at that time of the observations with a dozen sets of clever solar instruments. European and US scientific teams contributed the instruments to this project of international cooperation between ESA and NASA.

"What started as unusual research has become an everyday tool," notes Jean-Loup Bertaux of the CNRS Service d'Aeronomie near Paris, who leads the French-Finnish team responsible for the SWAN instrument. "We should no longer be taken by surprise by highly active regions that suddenly come into view as the Sun rotates."

The Sun takes roughly four weeks to turn completely around on its axis, but active regions can appear and grow in only a few days. So until two years ago, no one had any way of telling when an active region might come 'around the corner' - perhaps blazing away with eruptions as soon as it appeared. If an active region can be detected in the middle of the far side it will appear on the eastern (left-hand) side of the visible disk about seven days later. The SWAN team announced the telltale ultraviolet observations in June 1999.

In March 2000 Charles Lindsey of Tucson, Arizona, and Doug Braun of Boulder, Colorado, reported that they had detected, with SOHO's MDI, sound waves reflected from far-side sunspots. Speeded by the intense magnetic fields associated with sunspot regions, the sound waves arrived a few seconds early at the Sun's near-side face, compared with sound waves from sunspot-free regions. Decoding MDI data from a million points on the Sun's near side, to obtain an impression of the far side, uses a technique called helioseismic holography and requires a powerful computer.

Both discoveries were made retrospectively from SOHO's archives. Since then teams have streamlined their data gathering and analyses to the point where they can offer routine long-range forecasts of intense solar activity based on far-side foresight. The techniques are complementary, with MDI seeing the sunspot regions and SWAN reporting how active they are.

"When we started work with SOHO five years ago, most experts thought it would be impossible to see right through the Sun," comments Philip Scherrer of Stanford University, principal investigator for the MDI instrument. "Now we do it regularly in real time. For practical purposes we've made the Sun transparent."

Although conceived for scientific research, SOHO has proved invaluable as a watchdog for spotting sunstorms. Forecasters already rely heavily on SOHO's round-the-clock observations of flares and mass ejections that can have harmful effects on satellites, power lines and other technological systems. The new long-range, far-side forecasts may be especially useful for scheduling manned space operations, during which astronauts might be exposed to dangerous particles from solar explosions.

Watching the solar striptease
SOHO examines the Sun from a vantage point 1.5 million kilometres out, on the sunward side of the Earth. Its instruments probe the Sun from its nuclear core, through its turbulent interior and stormy atmosphere, and all the way out to the Earth's orbit and beyond, where a non-stop stream of atomic nuclei and electrons travels outwards as the solar wind. To the naked eye the Sun looks calm and unchanging, but for SOHO it has performed a dramatic striptease. Here are just ten of the revelations.

The Sun's surprising heart beat. Currents of gas far beneath the visible surface speed up and slacken again every 16 months - a wholly unexpected pulse-rate. It was detected by combining data from SOHO and a US-led network of ground stations called GONG.

Brighter sunbeams. Watching minute by minute and year by year, SOHO has seen the Sun brighten, as expected, by 0.1 per cent while the count of sunspots increased during 1996-2000. By studying the variations in detail, scientists estimate that high-energy ultraviolet rays from the Sun have become 3 per cent stronger over the past 300 years.

Eruptions coming our way. Most of the explosive outbursts of gas from the Sun, called coronal mass ejections, miss the Earth. Only SOHO can reliably identify those heading in our direction, by linking expanding haloes around the Sun to shocks seen in the Earth-facing atmosphere. Engineers then have 2-3 days' warning of possible effects in the Earth's vicinity.

Thousands of explosions every day. A reason why the Sun's atmosphere is far hotter than its visible surface is a non-stop succession of small explosions, observed by SOHO. They result from a continual rearrangement of tangled magnetic fields.

The sources of the solar wind. SOHO sees gas leaking from the corners of a magnetic honeycomb of gas bubbles, mainly in polar regions, to supply a fast solar wind. Nearer the Sun's equator, a slow wind escapes from the edges of wedge-shaped features called helmets.

Accelerating the solar wind. Charged atoms feeding the fast wind gain speed very rapidly - evidently driven by strong magnetic waves in the Sun's outer atmosphere. Similar magnetic waves may accelerate the slow wind too, although many mass ejections also contribute to it.

Elements in the solar wind. SOHO detected phosphorus, chlorine, potassium, titanium, chromium and nickel for the first time, and previously unseen isotopes of six commoner elements. These give clues to conditions on the Sun, and also to Solar-System history.

Gigantic sunquakes. After a solar flare, SOHO sees waves rushing across the Sun's visible surface, like the ripples seen when a stone falls into a pond. One such event was judged to be 40 000 times more energetic than the San Francisco earthquake of 1906.

Huge solar tornadoes. SOHO discovered tornadoes as wide as Africa, with hot gas spiralling outwards from the polar regions of the Sun. Typical wind speeds of 50 000 kilometres per hour can become ten times faster in gusts.

The alien breeze. A wind of gas from the stars blows through the Solar System, and the solar wind fights it. SOHO has fixed its direction (from the Ophiuchus constellation) and its speed (21 km/s) more accurately.

Some facts and figures about SOHO
With scientists from 62 institutes in 15 countries, in the teams that provide and operate the instruments, and with industries in 15 countries contributing to the spacecraft's construction, SOHO is a masterpiece of international collaboration.

Weighing 1.85 tonnes at launch, the European-built SOHO was dispatched by a NASA rocket on 2 December 1995, and transferred to the vicinity of Lagrange Point No. 1, where it now hovers, 1.5 million kilometres from the Earth.

The spacecraft was commissioned in April 1996 for a nominal operational life of two years, but this was later extended by five years until the end of March 2003.

Observations were severely interrupted twice, between 25 June and 5 November 1998, and between 21 December 1998 and 2 February 1999. The first event was due to loss of contact and control, and the second to gyroscope failure. In both cases ESA and NASA engineers, fully supported by SOHO's constructor Matra Marconi, worked wonders to restore the spacecraft to full operations.

More than 30 eruptions called solar proton events have bombarded SOHO with energetic particles. The most severe, on 14 July and 9 November 2000, temporarily blinded SOHO's instruments with particle 'snow' and slightly impaired the efficiency of the spacecraft's power-generating solar panels.

More than 3600 coronal mass ejections from the Sun have been observed by SOHO's LASCO instrument, making an average of two per day during SOHO's 5 years of observations.

SOHO is by far the most prolific discoverer of new comets in the entire history of astronomy. By mid-April 2001 the number stood at 304, most of them being small comets that fall into the Sun. Amateur astronomers around the world examine SOHO's daily pictures, via the Internet, and have been first to spot more than 200 of the SOHO comets.

The scientific payoff from SOHO is apparent in more than 2000 papers, theses and reports, to which more than 1400 individual researchers have contributed.

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